Sewing by a sewing machine is performed while a cloth is held by a presser foot. The presser foot has a primary function of suppressing, when a needle is pulled out from a cloth, an uplifting of the cloth associated with the pulled-out needle. In addition, the presser foot has a secondary function of holding the cloth together with a feed dog, and ensuring a cloth feeding. When, however, a stitchwork formation is performed on the cloth, the cloth is held by a stitchwork frame, and the cloth is translated in vertical and horizontal directions by a frame driving mechanism, and feed dog is not utilized at this time. The presser foot at the time of stitchwork formation is mainly utilized to suppress an uplifting of the cloth associated with the pulled-out needle.
Pushing the cloth so as to contact with the feed dog by the presser foot disturbs an operation of the frame driving mechanism. In addition, frictional damages are likely to be formed on the cloth and a stitchwork pattern. In order to suppress an uplifting of the cloth, it is sufficient if the presser foot is positioned so as to be slightly apart from the surface of the cloth. Hence, according to conventional technologies, when a sewing machine is utilized for a stitchwork formation, the presser foot is lifted up by a predetermined distance from the surface of the cloth before a stitchwork formation starts, and then the stitchwork formation is started (see, for example, JP2006-20757 A).
The presser foot is attached to a presser bar that is capable of moving up and down. Before a stitchwork formation starts, the presser bar is moved down to cause the presser foot to be once in contact with a cloth, and the presser bar is moved up by a predetermined distance from the contact position. Next, the presser foot is fixed at a height where the presser bar completes the move-up operation, and then the stitchwork formation is started.
According to JP2006-20757 A, the height of the presser foot that has been set initially at the beginning of the stitchwork formation is always maintained during the stitchwork formation. Depending on a pattern to be stitched, however, a thread may overlap several times relative to the cloth. A pattern to be formed on the cloth increases the thickness along with the advancement of the stitchwork formation, and the clearance below the presser foot becomes narrower than the predetermined distance that has been set initially.
When the clearance below the presser foot becomes narrow, the presser foot may contact the pattern. In this case, an improper sewing, such as frictional damages to the cloth and the stitchwork pattern, may occur, decreasing a quality. Conversely, when the height of the presser foot is initially set in consideration of the thickness of the pattern to be formed beforehand, the clearance between the surface of the cloth and the presser foot is too wide at the beginning of a stitchwork formation, and thus the presser foot is unable to surely suppress an uplifting of the cloth.
The present invention has been proposed to address the above-explained technical problems of conventional technologies, and it is an objective of the present invention to provide a sewing machine that does not deteriorate qualities of a cloth and a stitchwork pattern even if a thickness of a stitchwork pattern increases along with an advancement of a stitchwork formation.